1. Field of the Invention
The present invention relates to a wire electric discharge machine capable of detecting a machining state without being easily influenced by variation in a machining-gap voltage detection circuit in each machine.
2. Description of the Related Art
In a wire electric discharge machine, a voltage is applied to a machining gap between a wire electrode and a workpiece opposite each other to generate electric discharge. At the same time, the relative positions of the wire electrode and the workpiece are changed so that the workpiece is machined into a desired shape. Since discharge craters produced by electric discharge are collected to form a machined surface, the surface roughness of the workpiece depends on the size of each individual discharge crater. It is known, therefore, that a good machined surface can be obtained by applying a high-frequency voltage to the machining gap and frequently repeating electric discharge of a short time duration. For example, Japanese Patent Application Laid-Open No. 61-260915 discloses how a machined surface with a surface roughness of 1 μm Rmax or less can be obtained by machining a workpiece by applying a high-frequency AC voltage of 1 to 5 MHz to a machining gap.
Finish machining is expected to correct the shape of a rough-machined workpiece more precisely. Those portions which are made larger than a target size after rough machining need to be control led so that the machining amount increases during finish machining. In contrast, those portions which are made smaller than the target size after rough machining need to be controlled so that the machining amount is reduced after finish machining. Specifically, it is necessary to determine according to the machining state whether or not the target size is exceeded during finish machining. Based on the result of this detection, the relative positions of the wire electrode and the workpiece and machining conditions need to be controlled or changed.
FIG. 7 shows a typical high-frequency voltage waveform. As shown in (1) of FIG. 7, a necessary minimum dead time is provided between each two adjacent cycles of voltage application so that a bridge circuit in a power supply circuit is not shorted. A machining-gap voltage has a sinusoidal waveform, as shown in (3) of FIG. 7.
The average of the absolute value of a machining-gap voltage is the most typical index indicative of the machining state. Since an average machining-gap voltage substantially represents the distance of a gap between a wire electrode and a workpiece, a high-precision machining shape can be obtained by controlling axis feed in a manner such that the average machining-gap voltage is constant. If finish machining is performed by applying the high-frequency voltage shown in FIG. 7 to the machining gap, in order to obtain a satisfactory surface roughness, however, the response of a machining-gap voltage detection circuit is degraded in a high-frequency region, so that it is difficult to obtain accurate average, voltage data. Since variation in the component characteristics of the detection circuit is substantial in the high-frequency region, moreover, detected values vary according to the machine. If the axis feed control is performed based on such data, the result of machining inevitably varies depending on the machine.
To overcome this, International Publication No. 2004/022275 discloses a technique in which a DC voltage is superposed on an AC high-frequency voltage to be applied, and only a low-frequency voltage component of a machining-gap voltage is extracted by means of a low-pass filter. The feed of a wire electrode is controlled according to the change of the extracted voltage component. Since the average voltage cannot be reduced to zero according to this technique, electrolytic corrosion may possibly occur in a workpiece or machine body. Since the low-pass filter is used, moreover, the response is too poor to enable follow-up in case of a sudden change of the electric-discharge state.
The frequency of electric discharge per unit time is an index other than the average voltage. Japanese Patent Application Laid-Open No. 2002-254250 discloses a technique for controlling the axis feed rate and quiescent time based on the frequency of electric discharge. To this end, the machining-gap state needs to be classified into three states; open-circuit state, electric-discharge state, and short-circuit state. In the open-circuit state, electric discharge is not performed at all after the voltage application. In the open-circuit state, the wire electrode and the workpiece contact each other so that electric discharge does not occur. Thus, machining is not effected in either of these states.
If a high-frequency voltage of a sinusoidal waveform is applied, as shown in FIG. 7, however, an application time in each duty cycle (application time plus dead time) is very long ((1) of FIG. 7). Even if electric discharge occurs, therefore, a voltage is applied immediately when insulation in the machining gap is recovered, so that the machining-gap voltage inevitably increases. Consequently, the time during which the machining-gap voltage is actually reduced becomes short. The detection circuit cannot respond to such a very high frequency, so that the electric-discharge and open-circuit states cannot be distinguished from each other.
Thus, in the prior art, the detected machining-gap voltage is the only factor that can be used as an index for a high-frequency machining state. In the high-frequency region, however, the detected value of the machining-gap voltage varies according to the wire electric discharge machine, under the influence of the variation in the response and component characteristics of the detection circuit. Thus, the result of machining inevitably varies.